Question

procedure c:

• the percentage of minerals within specific igneous rocks

5: at what temperature does melting begin for ultramafic rocks?
6: approximately what percentage of silica is in ultramafic rocks?
7: is the plagioclase feldspar found in granite more sodium - rich or calcium - rich?
7b: making connections: what does this tell you about the temperature at which the magma cooled to form granite?

Explanation:

1. Refer to the bottom temperature axis of the graph; ultramafic rocks correspond to the rightmost range, where melting begins at the lower end of this range.
2. Locate the silica percentage axis (bottom gray bar) and match it to the ultramafic rock category.
3. Granite is a felsic rock; the graph shows felsic plagioclase is sodium-rich (albite endmember).

7b. Sodium-rich plagioclase forms in lower-temperature environments, so granite magma cooled slowly at lower temperatures.

Answer:

1. $\boldsymbol{1200^\circ C}$
2. $\boldsymbol{45\%}$ (approximate range 40-50% is acceptable)
3. More sodium-rich

7b. The magma that formed granite cooled at a relatively low temperature, as sodium-rich plagioclase crystallizes at lower temperatures compared to calcium-rich plagioclase.